The invention relates to a mercury-free metal halide lamp according to the preamble of Claim 1. It relates in particular to lamps for the luminous colour warm white (WDL) for general illumination, which in particular is dimmable.
DE-A 197 31 168 has already disclosed a mercury-free metal halide lamp which uses two groups of metal halides, namely voltage generators, which primarily take over the role of the mercury, and light generators, in particular rare-earth elements. Therefore, warm white luminous colours of around 3500 K are desired. However, the red reproduction remains unsatisfactory, which is controlled by adding metal halides of Dy or Al. Similar fill systems are also described in WO 99/05699 and EP-A-833,160.
WO 98/45872 describes a mercury-containing metal halide lamp, the fill of which essentially contains Na and Tl-containing metal halides. In addition there are Dy metal halides and Ca metal halides. This fill is aimed at a neutral white luminous colour of 3900 to 4200 K.
When producing warm white and neutral white luminous colours, the use of sodium is disadvantageous, since it diffuses readily due to its small ionic radius.
The object of the present invention is to provide a metal halide lamp according to the preamble of Claim 1 which not only does not contain any mercury, for environmental reasons, but also completely or as far as possible avoids the use of sodium, in order to bypass the associated well-known difficulties. In particular, this affects the construction of lamps which are capped on one side (problem of photoionization).
This object is achieved by means of the characterizing features of Claim 1. Particularly advantageous configurations are given in the dependent claims.
According to the invention, a mercury-free metal halide lamp with a warm white luminous colour and a high colour rendering index Ra is proposed, the lamp comprising a discharge vessel into which electrodes are introduced in a vacuum-tight manner and with an ionizable fill in the discharge vessel. The fill comprises the following components: an inert gas which acts as a buffer gas, a first group of metal halides (MH), the boiling point of which is over 1000xc2x0 C. (preferably above 1150xc2x0 C.), the first group comprising at least Dy and Ca used simultaneously as metals, and the molar ratio of the two metal halides Ca-MH:Dy-MH being between 0.1 and 10, preferably between 0.2 and 5; these are components with a low volatility which are present in saturated form; a second group of metal halides, the boiling point of which is below 1000xc2x0 C. (preferably below 900xc2x0 C.), the second group comprising at least one of the elements In, Zn, Hf, Zr as metals; these are volatile components which are mostly present in unsaturated form; the total fill quantity of the first group of metal halides being between 5 and 100 xcexcmol/cm3; the total fill quantity of the second group of metal halides being between 1 and 50 xcexcmol/cm3; the colour temperature being between 2700 and 3500 K; the general colour rendering index being at least Ra=90, while at the same time the red rendering index is at least R9=60.
Preferably, the molar ratio of the two metal halides Ca-MH:Dy-MH is between 0.3 and 4. The second group preferably additionally comprises a metal halide of Tl in an amount of up to 30 xcexcmol/cm3, preferably 5 to 25 xcexcmol/cm3.
Furthermore, the first group may also include a metal halide of Na in a proportion of up to 30 mol %, preferably of at most 5 mol %, of the total quantity.
Preferably, the first group additionally comprises a metal halide of Cs in an amount of up to 40 xcexcmol/cm3, preferably 5 to 30 xcexcmol/cm3. Moreover, the cold filling pressure of the inert gas is advantageously between 100 and 10,000 mbar.
The members of the second group may additionally be added as metals in a proportion of up to 30 mol %. Moreover, at least one elemental metal or a metal halide of the metals Al, Ga, Sn, Mg, Mn, Sb, Bi, Sc may additionally be added to the second group, specifically in total in an additional proportion of up to 40 mol %.
Furthermore, at least one metal halide of the metals Sr, Ba, Li and/or the rare-earth elements may additionally be added to the first group, specifically in total in an additional proportion of up to 30 mol %.
Preferably, the discharge vessel is ceramic and has a typical ratio of the maximum internal longitudinal/lateral dimensions of at most 3.5.
Advantageously, the dimensioning of the inner wall surface is selected in such a way that, in operation, an internal wall loading of 10 to 60 W/cm2 prevails.
The Hg-free fill is essentially an Na-depleted fill (preferably at most 5 mol % Na halide in the fill proportion with a boiling point of  greater than 1000xc2x0 C.). Its composition is selected in such a way that at least Dy halide and Ca halide are included as fill constituents in the proportion of the fill substances with a boiling point of  greater than 1000xc2x0 C., and that at least one metal halide MH with a boiling point of  less than 1000xc2x0 C. which is selected from the group In, Zn, Hf, Zr is included.
Particularly if the ratio Ca-MH/Dy-MH is  greater than 2 (in particular  greater than 4), it may be advantageous to add further metal halides to the fill, preferably the lanthanides listed below, in a proportion of up to 25 mol %, in order to compensate for the overhang in the red spectral region, caused by the CaI2 content.
The total fill quantity of the first group in the discharge vessel is to amount to CaX2+DyX3=5-100 xcexcmol/ccm (X is any desired halide selected from I, Br and Cl). Furthermore, the total fill quantity of the second group, relating to metal halides MeXn of the metals In, Zn, Hf, Zr is in total to be MeXn=1-50 xcexcmol/cm3. If this parameter is selected at a lower level, the voltage gradient is below 50 V/cm, which is not practical.
Preferably, the addition of Tl-MH is in the range TlX=5-30 xcexcmol/ccm. The optimum quantity is to be selected as a function of other constituents in order to achieve the smallest possible deviation from the Planckian locus.
The spectral emission of the light source is in the warm white spectral region between 2700 K and 3500 K, and the general colour rendering index is preferably Ra greater than 90, the red rendering index of saturated red being R9 greater than 60.
A particularly noteworthy feature of the present invention is that excellent constancy of the colour rendering is maintained even when the lamp is dimmed to approx. 50% of the lamp output. Previous fills were unsuitable for dimming. This is due to the balanced mix between Dy and Ca in combination with the possibility of enriching the Ca (and if appropriate also Cs) in the vapour phase by molecule formation (formation of complexes). This mechanism is particularly effective in mercury-free fills. As a result, the output is made independent of the spectral emission distribution in the visible spectral region, corresponding to an excellent dimmability.
The lamp fill may contain Cs halide in the fill constituent of the fill substances with a boiling point of  greater than 1000xc2x0 C., in a mol % concentration of preferably between 10-50%, the total amount of the CsX typically being between 5-40 xcexcmol/cm3. This is because CsX improves the arc stability and increases the light efficiency.
In addition, the lamp fill may contain at least one metal halide with a boiling point of  less than 1000xc2x0 C. which is derived from the group Al, Ga, Sn, Mg, Mn, Sb, Bi, Sc. These substances can be added to the mixture in order to precisely set the voltage; some substances are also suitable for influencing the spectral emission distribution.
In a further embodiment, the lamp fill may additionally contain at least one elemental metal from the group Tl, In, Zn, Al, Ga, Sn, Mg, Mn, Sb, Bi, Sc, the fill quantity lying in the range between 0.5-50 xcexcmol/cm3. These substances can be added to the mixture in order to improve the electrical performance, for example to minimize restarting peaks.
The optional proportion of Na halide may amount to up to 30 mol % of the proportion of the fill constituents which have a boiling point of  greater than 1000xc2x0 C. Although NaI typically impairs the dimming performance or constancy of the colour rendering, it may also be added in order to increase the light efficiency.
In a further preferred embodiment, at least one halide of the lanthanides and from the group Sr and Ba and Li may be included in the fill proportion with a boiling point of  greater than 1000xc2x0 C., typically in a molar concentration of up to 35 mol %. These substances are added to the mixture in order to optimize the spectral distribution in the visual spectral region, e.g.: Sr, Ba and Li for further improvement of the emission in the red spectral region, lanthanides in the blue spectral region and green spectral region.
Preferably, the ionizable fill comprises at least one inert gas (Ar, Kr, Xe) with a cold filling pressure of 100-10,000 mbar. An extended service life is in particular possible with a cold filling pressure of typically more than 500 mbar Ar. Below 100 mbar, electrode loading is too high during the starting phase of the lamp, leading to a poor maintenance performance.